A simple goiter can occur without a known reason. It can occur when the thyroid gland is not able to make enough thyroid hormone to meet the body's needs. This can be due to a lack of iodine in a person's diet. To make up for the shortage of thyroid hormone, the thyroid gland grows larger.

Toxic nodular goiter is an enlarged thyroid gland that has a small, rounded growth or many growths called nodules. One or more of these nodules produce too much thyroid hormone.

The body needs iodine to produce thyroid hormone:

Simple goiters may occur in people who live in areas where the soil and water do not have enough iodine. People in these areas might not get enough iodine in their diet.

The use of iodized salt in many food products in the United States prevents a lack of iodine in the diet.

In many cases of simple goiter, the cause is unknown. Other than a lack of iodine, other factors that may lead to the condition include: